In a paper recycling process, wastepaper or what is commonly referred to as recovered paper is processed to remove contaminants such as glass, pieces of metal in the form of paper clips and wire staples, wax, toner, inks, adhesives, etc. The process begins with pulping where the recovered paper is mixed with water, with or without chemicals, to produce pulp. The pulp is then processed through various mechanisms to remove contaminants of all types and sizes. For example, the pulp can be passed through one or more screens, be processed in a hydrocyclone, be subjected to various flotation devices, be washed and sometimes even bleached. Other operations can also be applied to the pulp.
Adequate removal of contaminants from the pulp is necessary prior to using the pulp on a paper machine wherein the pulp is made into a certain grade of paper or paper board. For example, the pulp can be processed into a printing grade of paper, into a writing grade of paper, into coated paper, into newsprint, into tissue, into paper towels, into liner board, into a corrugated medium, into box board or into any other known kind of paper. One class of contaminants that causes serious problems in papermaking using recycled fibers is called stickies. Stickies arise from adhesives, glues, pressure sensitive adhesives, hot melt glues, binders such as those used in ink, coatings, waxes and related materials. Stickies in pulp affect the efficient operation of a paper machine as well as the quality of the finished product. It is estimated that the annual cost expended by paper mills, in the United States alone, is well over $700 million.
Stickies are classified into various categories based upon their size, properties, origin and behavior. One commonly used classification is based primarily on the size of the stickies. In this classification, stickies are divided into either macro stickies or micro stickies. Macro stickies are those retained on a slotted or mesh screen, such as a slotted screen where each slot has a width dimension ranging from between about 0.003 inches to about 0.006 inches, or a mesh screen where the mesh size ranges from between about 100 mesh to about 200 mesh. Micro stickies are those stickies that pass through the above-identified screens.
Various methods can be used to measure the amount, size, concentration or other parameters of the macro stickies in a dry or wet pulp sample or in a slurry including pulp, water and other ingredients. In one example, a pulp sample is processed through a test device, such as a Pulmac Master Screen, manufactured by Pulmac Inc. of Quebec, Canada, where a slotted screen is used to separate macro stickies from pulp fibers, micro stickies, fines and ash. However, the Pulmac Master Screen currently is not able to measure micro stickies. There are currently several known methods for measuring micro stickies but most are either cumbersome to carry out, require an exorbitant amount of time to complete the test, require the test sample to be refrigerated for a period of time or are costly to perform. In addition, a number of these methods are not very effective in separating the macro stickies from the micro stickies thereby producing erroneous test results.
Now a method has been invented which is capable of measuring macro and micro stickies in a recycled sample containing pulp fibers or in process water containing pulp fibers. This method is less costly, more efficient and can be conducted in a matter of minutes. In addition, this method produces reliable test results for both the macro stickies and the micro stickies.